T-independent (TI) B cell responses are critical for the production of antibodies to bacterial capsular polysaccharides (TI-2 antigens), and thus play an important role in anti-bacterial immune responses. It has been proposed that these responses are mediated by immunoglobulin (Ig) receptor cross-linking by repeating determinants on the polysaccharides, Certain strains of mice with an X chromosome linked genetic defect, xid, do not mount such antibody responses. These mice are reported to be missing a subset of B cells, the Lyb-5+ cells, proposed to be required for TI B cell responses. Serum levels of certain immunoglobulin isotopes produced by T independent B cell responses, IgG3 and IgM, are lower in xid mice relative to otherwise genetically identical non-defective mice. Conversely, IgA levels are higher in xid mice. It has been suggested that Lyb-5- cells are less able to switch to and produce the IgG3 isotype. Our data demonstrates that a third isotype, IgG2b, is over-expressed in some xid mice. We also have found using an LPS culture system that directs switching to IgG3 and IgG2b that B cells from xid mice do not have an impairment in switching to or production of IgG3, nor do they have a propensity to switch to IgG2b. Thus, it is more likely that the low IgM and IgG3 is caused by a lack of response of xid B cells to environmental stimulation by bacterial TI-2 antigens. We are currently investigating the basis for the elevated expression of IgG2b and IgA. It has been reported previously that B cells from xid mice differ from normal B cells in usage of immunoglobulin V regions. Our lab has demonstrated using a B cell colony filter disk hybridization assay that although V region usage is on the average similar between xid and normal mice, variation between mice is evident in xid but not normal B cells. We are currently investigating the genetic basis of this phenomenon.